This invention relates generally to locking screws, and more particularly to a screw for use with a pre-tapped mating nut or other threaded hole and wherein good resistance to vibrational loosening is required.
The present invention is particularly applicable to a screw for use with a pre-tapped nut or like threaded member and wherein the pre-tapped thread is likely to be partially coated with paint or other contaminants. Permanently attached nuts on automobiles or other vehicles or tapped holes for containers are typical examples of the pre-tapped threads which are likely to be contaminated and with which the screw of the present invention is intended to be used.
Various types of screws are known for use with pre-tapped holes. Generally speaking those screws have a substantially circular cross-section and are free running provided the nut threads are clean. If resistance to vibrational loosening is required these screws of circular cross-sections can be modified by the addition of plastic (such as nylon) or other non-metallic patches over a portion of their length. If the pre-tapped nut or female thread is contaminated with paint or other hardened material, difficulty is sometimes encountered in using screws of the type which are of a circular cross-section with or without the addition of non-metallic or plastic patches.
Another known type of screw for use with a pre-tapped hole has a trilobular cross-section but wherein the thread includes a double angle profile. In this type of thread profile the flanks of the thread from the root of the thread outwardly past the pitch circle are at the normal flank angle. However, at the crest portion of the thread the flank angle is reduced which causes an interference between the thread on the screw and the thread in the hole. For example, in one known arrangement the flanks are at the normal 60 degree angle from the root of the thread outwardly a certain distance past the pitch circle, but at the crest region on of the thread, the flank angle is about 30 degrees. In a trilobular screw of the type having the 60 degree-30 degree double angle profile the screw creates a resistance to vibrational loosening.
Also known are several types of screws designed for removing female thread contamination as they are inserted. However, none of these screws are intended to provide a locking arrangement which resists vibrational loosening. In another known arrangement an otherwise known trilobular screw was modified by having its entry portion almost triangular in configuration. It was intended that the triangular entry portion would create a scraping effect while at the same time providing a clearance space for contaminants scraped off of the female thread. Such an arrangement was found to be successful in cases where there were small amounts of fairly soft contamination material. In recent years, however, automobile paints have become much harder and much thicker in order to protect the vehicle against corrosion. Where the threaded hole is contaminated by such coatings and a locking screw of the double angle type is used, the extra surface pressures generated by the hard contaminants can cause excessive and objectionable noise levels as the screws are driven into the contaminated holes. Moreover, the contaminant adversely effects the torque-tension relationships in driving the screw into the hole, and in extreme cases might possibly cause failure of the screw.